Air, Water Vapor, and Aerosol Transport through Textiles with Surface

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Surfaces, Interfaces, and Applications

Air, Water Vapor, and Aerosol Transport through Textiles with SurfaceFunctional Coatings of Metal Oxides and Metal Organic Frameworks Natalie Pomerantz, Erin Anderson, Nicholas Dugan, Nicole Hoffman, Heather F. Barton, Dennis T. Lee, Christopher J Oldham, Gregory W. Peterson, and Gregory N. Parsons ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b04091 • Publication Date (Web): 05 Jun 2019 Downloaded from http://pubs.acs.org on June 5, 2019

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ACS Applied Materials & Interfaces

Air, Water Vapor, and Aerosol Transport through Textiles with Surface-Functional Coatings of Metal Oxides and Metal Organic Frameworks Natalie L. Pomerantz1*, Erin E. Anderson2, Nicholas P. Dugan1, Nicole F. Hoffman1, Heather F. Barton3, Dennis T. Lee3, Christopher J. Oldham3, Gregory W. Peterson4, Gregory N. Parsons3

1US

Army Combat Capabilities Development Command Soldier Center, 10 General Greene

Ave. Natick, MA 01760

2Battelle

3North

4US

Memorial Institute Natick Operations, 313 Speen St, Natick, MA 01760

Carolina State University, 911 Partners Way, Raleigh, NC 27606

Army Combat Capabilities Development Command Chemical and Biological Center, 5183

Blackhawk Rd, Aberdeen Proving Ground, MD 21010

KEYWORDS atomic layer deposition, metal organic frameworks, aerosol transport, filtration, chemical/biological protection, breathable materials ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT: Currently, air permeable Chemical/Biological (CB) protective garments are based on activated carbon technology, which reduces moisture vapor transport needed for evaporative cooling and has potential to absorb and concentrate toxic materials. Researchers are exploring classes of sorbent materials that can selectively accumulate and decompose target compounds for potential to enhance protective suits and allow for novel filtration devices. Here, the metal-organic frameworks (MOFs) UiO-66-NH2 and HKUST-1 have been identified as such materials. In order to better understand how MOFs can perform in future CB protective systems, atomic layer deposition (ALD) and solution deposition were used to modify nonwoven polypropylene and flame resistant fabrics with HKUST-1 and UiO-66-NH2. Air permeation, water vapor transport, filtration efficiency, and chemical reactivity against chemical agent simulants were assessed in relation to ALD thickness and MOF crystal size. MOF deposition on substrates decreased both air and chemical permeation, while increasing filtration efficiency and chemical sorption. Moisture vapor transport was not affected by MOF growth on substrates, which is promising when considering thermal properties of protective garments. Future work should continue to explore how MOF deposition onto fiber and textile substrates impacts transport properties and chemical absorbance. ACS Paragon Plus Environment

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INTRODUCTION

Current air permeable Chemical/Biological (CB) protective garments are based on activated carbon technology. Activated carbon is a cheap, non-selective sorbent which is easily processed and incorporated into CB protective textiles to offer protection against a variety of threats. However, carbon also reduces the moisture vapor transport (MVTR) through fabric composites1. A high MVTR is essential to maintaining evaporative cooling from the skin, which reduces the chance of heat injury and increases the work/rest cycle of first responders and the military2. Other shortcomings with carbon include durability and susceptibility to poisoning by battlefield contaminants3. Another concern, particularly for toxic compounds, is that absorbed material becomes concentrated within the carbon, causing problems for handling and decontamination4. Laundering also reduces the effectiveness of the carbon within the suits and the service life is dictated not by the durability of the garment itself, but by how many times the suit is laundered5. Novel sorbents and reactive materials have the potential to address the shortcomings of carbon

ACS Paragon Plus Environment

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as a CB protective material by not reducing the MVTR, and thereby decreasing the thermal burden on the wearers, and not being susceptible to contamination, thus increasing the service life, durability and shelf life.

To address these concerns, researchers are exploring new classes of sorbent materials that can selectively accumulate and decompose target compounds. For example, select metal organic frameworks (MOFs) have been shown to be excellent sorbent materials which are stable for long durations and under a variety of conditions6. More recently, the HKUST-1, UiO-66, UiO-66-NH2 and UiO-67 MOFs have been shown to be sorptive and reactive to chemical warfare agents (CWAs) and toxic industrial chemicals (TICs)7. A variety of techniques have been investigated for incorporating MOFs into (and onto) fibers and textiles. For example, Wang and coworkers demonstrated the ability to electrospin composites of ZIF-8, UiO-66-NH2, HKUST-1, and Mg-MOF-74 in poly(acrylonitrile) and polystyrene8. Direct growth of UiO-66-NH2 on poly(acrylonitrile) nanofibers using a precursor seeding strategy was also explored, resulting in high MOF loadings on the fiber9. Other techniques such as using reactive dye chemistry has also enabled MOF growth on


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cellulose10. Bromberg and coworkers incorporated NH2-MIL-101(Al) and NH2-MIL-53(Al) onto elastomeric surfaces using adhesives for use as self-detoxifying clothing11. Of these techniques, atomic layer deposition has shown the most promise in terms of high MOF loadings with uniform and conformal heterogeneous coverage on fibers while maintaining fiber morphology12,13.

The ability to develop sorbent-functionalized fabrics has the potential to not only enhance protective suits, but also allow for novel filtration devices. Using this approach, multiple MOFs can be grown on fibrous layers, which can in turn be used to develop new, conformal filters14. Each layer could contain MOFs with highly specific efficiency towards certain chemicals, such as ammonia, chlorine, nitrogen dioxide, and nerve agents, resulting in a broad spectrum filtration device6.

In order to better understand how MOFs on fibers can perform in future CB protection systems, more work is needed to define fundamental relationships on how fiber surface modification (i.e. ALD and MOF coatings) influence vapor species transport into and through MOF-textiles. This article addresses this need by exploring and analyzing the


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effect of HKUST-1 and UiO-66-NH2 MOF crystal growth on air permeation and water vapor transport through nonwoven polypropylene.

EXPERIMENTAL

Nonwoven polypropylene (PP) microfiber substrates were used as received from the Nonwovens Institute at NC State University. PP fibers ranged from 2 to 20 μm, with an average diameter of 10 μm. Fabric swatches weighed 0.004 g/ cm2 with a surface area of 1.5 m2/g as measured by nitrogen isotherms at 77 oC. Fabric samples consisted of a woven flame resistant (FR) material currently in use by the US military. The FR fabric is a woven fabric blend blend of consisting mostly of aramid and FR rayon. Commercially available carbon based protective materials were purchased and evaluated for comparison with the developmental materials and to demonstrate the range of performance available in military protective fabrics. Carbon based material 1 is a composite consisting of a woven cover fabric and a layer of carbon beads in a polymer mesh. Carbon based material 2 is an activated carbon cloth. Carbon based material 3


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consists of two cotton knit layers with carbon beads adhered in between the layers. Aerosol protective layer 1 is similar to carbon based material 3 but also includes polyurethane nanofibers in the fabric composite. Aerosol protective layer 2 is similar to aerosol protective layer 1 but has less nanofibers.

ATOMIC LAYER DEPOSITION OF ZnO OR TiO2 ON POLYPROPYLENE OR FR FABRIC

Zinc oxide (ZnO) and titanium dioxide (TiO2) ALD coatings were deposited in a hot-wall viscous-flow reactor. The deposition pressure and temperature were ~1.8 Torr and 90 °C, respectively. In an ALD TiO2 cycle, precursors of both titanium tetrachloride (TiCl4, Strem Chemicals) and water were dosed alternatively to the reaction chamber for 1 sec, with 40 and 60 sec of N2 purge gas following the TiCl4 and water dose steps, respectively. 300 cycles of ALD TiO2 were deposited onto either PP, or onto FR fabric to obtain a TiO2 thin film on the substrates. In an ALD ZnO cycle, substrates were exposed to 2 sec of di-ethyl zinc (DEZ, Strem Chemicals) or deionized water in alternate steps with 60 sec of N2 purge


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between dose steps. 100, 200, or 300 cycles of ALD ZnO were deposited on PP. ALD film deposition thickness was monitored using a J. A. Woollam Co. ellipsometer to measure 1x1 cm2 silicon wafers placed in each run.

HKUST-1 ON ALD ZnO COATED POLYPROPYLENE

The HKUST-1 synthesis precursors included 8.7 g (36.0 mmol) of cupric nitrate trihydrate (Cu(NO3)2•3H2O, puriss. p.a., 99-104%, Sigma) in a 240 mL solution of 1:1 deionized water : dimethylformamide (DMF, Fisher), and 4.2 g (20.0 mmol) 1,3,5-benzenetricarboxylic acid (H3BTC, 95%, Sigma) in 120 mL anhydrous ethanol (Koptec). The solutions were combined and magnetically stirred in a beaker at room temperature for 2 min, then transferred to an 8 x 10 x 2 in. Pyrex baking dish. A 7 x 8 in. ZnO coated PP microfiber substrate was submerged in the solution mixture for 2, 4, or 6 min corresponding to 100, 200, or 300 cycles of ALD ZnO. Samples were then washed in methanol (Fisher Chemical) for 24 h, followed by vacuum drying at 85 °C for 24 h to thermally activate the material.


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UiO-66-NH2 ON ALD TiO2 COATED POLYPROPYLENE OR FR FABRICS

Precursor solution for UiO-66-NH2 growth on TiO2 coated PP or FR fabric was prepared with 0.80 g (3.43 mmol) of zirconium chloride (ZrCl4, ≥99.5%, Alfa Aesar) in 200 mL of DMF in a cylindrical glass bottle, followed with 1 min of sonication and 5 min of magnetic stirring to dissolve ZrCl4 well. 0.62 g (3.43 mmol) of 2-aminoterephthalic acid (H2BDCNH2, 99%, Sigma) was then added to the prepared ZrCl4 solution under stirring. After that, 250 µL of deionized water and 1.33 mL of concentrated hydrochloric acid (NF/FCC grade, Fisher) were added to the mixed solution in a consecutive order. The solution was transferred into a Pyrex glass (7.5” × 5.5” × 2.5”), followed by immersing a free-standing fabric sample (7” × 5”) in it. The Pyrex with sample was then placed in a vacuum oven (Fisher ScientificTM IsotempTM) and heated to 85 °C for 24 h. After the solvothermal synthesis, the MOF-coated substrate (PP or FR fabric) was soaked with 100 mL of DMF in a Pyrex for at least 12 h, and the solvent was replaced every 6 h. After DMF wash, the MOF-coated fabric was further rinsed by being immersed in 100 mL of anhydrous ethanol


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and the solvent was replaced every 12 h for a total of three times. For further characterizations, the MOF-coated fabric samples were thermally activated in the vacuum oven at 100 °C for 24 h under vacuum.

SEM photos were taken using an FEI Verios 460L. Samples were sputter coated with ~510 nm Au-Pd prior to SEM. XRD patterns were obtained using a Bragg-Brentano geometry on a Rigaku Smartlab X-ray diffractometer.

WATER VAPOR AND AIR TRANSPORT MEASUREMENT

The MVTR and air permeation were measured with the Dynamic Moisture Permeation Cell (DMPC), ASTM F229815. A flow cell configuration measuring a sample area of 0.001 m2 was used. The humidity of the feed and sweep streams were set to 95% and 5%, respectively, for a mean relative humidity of 50%. For porous, air permeable materials, the MVTR as a function of mean relative humidity is constant16. For semi-permeable membranes whose permeation is governed by solution-diffusion transport, the MVTR


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increases with mean relative humidity. As these materials are air permeable, it was decided that evaluating the MVTR at a single mean relative humidity would be sufficient to characterize the material.

In order to measure the MVTR of air permeable materials, the pressure drop across the sample was increased stepwise while the relative humidity of both feed and sweep streams remained constant. The MVTR was measured at each stepwise increase in pressure difference, and a linearization was used to determine the MVTR at ΔP = 0 by taking the intercept through the y axis. An example is shown in the Supplemental Information for a PP microfiber substrate coated with 200 cycles of ZnO ALD. The error for MVTR measurements was less than 5%15.

FILTRATION EFFICIENCY

The filtration efficiency of the materials was measured using a TSI Automated Filter Tester Model 3160 that measures particle penetration versus particle size at a set aerosol


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flow rate (face velocity). The TSI 3160 is capable of measuring efficiencies up to 99.999999%.

The TSI 3160 is equipped with two TSI Model 3772 Condensation Particle Counters (CPCs). The Model 3772 CPC detects particles as small as 0.010 microns in diameter and employs single-particle-count-mode operation to measure concentrations up to 10,000 particles per cubic centimeter. The detector counts individual pulses produced as each particle droplet passes through the sensing zone. A high signal-to-noise ratio and continuous, live-time coincidence correction provides accuracy even at very low concentration. The Model 3772 CPC uses a laser-diode light source and diode photodetector to collect scattered light from particles. The TSI 3160 is also equipped with a TSI Model 3302A Diluter to reduce the particle concentration of high-concentration aerosols. The Diluter was calibrated for dilution ratios of 100:1 and 20:1 at a total flow rate of 5 standard liters per minute.

For this test, the TSI 3160 generated aerosol particles in the range of 0.015 to 0.4 microns comprised of polydisperse dioctylphtalate (DOP, Sigma Aldrich) using an atomizer. Flow


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rates were set to achieve a sample face velocity of 5.0 cm/s. The 5.0 cm/s face velocity was selected based on Test Operations Procedure (TOP) 08-2-501A. The filtration efficiencies for the samples addressed in this paper were reported at a face velocity of approximately 5.4 cm/s (resulting from the 5.0 cm/s set point). Three replicate measurements on swatches approximately 5 inches in diameter were averaged for the reported values.

CHEMICAL ACTIVITY

Ammonia activity was determined via microbreakthrough testing. The system has been described previously17. Approximately 55 mm3 of composite to be tested was loaded into a fritted glass tube. The HKUST-1 MOF was tested either as powders, or once deposited by ALD onto PP. Ammonia was first charged to a ballast, which was then pressurized. This ballast was then mixed via mass flow controller with a diluent stream of clean air to achieve the required challenge concentration of 2,000 mg/m3. The effluent concentration was monitored with a photoionization detector equipped with a 10.6 eV lamp. Effluent


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data were integrated to determine the amount of chemical removed by each material, which was then used to calculate the loading per unit mass.

Permeation testing was conducted with 2-chloroethyl ethyl sulfide (CEES) according to ASTM F739-12. A swatch was placed over a 1 in. diameter Pesce PTC 700 permeation cell. Two countercurrent air streams at 300 mL/min at 0% RH (25 °C) were applied above and below the swatches. A concentration of 300 mg/m3 was fed to the challenge stream using a saturated vapor from liquid CEES within a saturator cell and mixing with a dry diluent stream. The feed, retentate, and permeate concentrations were all measured using a Hewlett-Packard 5890 Series II gas chromatograph equipped with flame ionization detectors.

RESULTS AND DISCUSSION

MATERIAL CHARACTERIZATION


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XRD indicated HKUST-1 was present on the PP fibers. Peaks at 6.7o, 9.5o, and 11.7o were clear on each HKUST-1 sample, matching the simulated PXRD pattern for HKUST1, shown in Figure 1. The HKUST-1 mass loading on 100, 200, and 300 cycles of ZnO was 14%, 28%, and 30%.

A

B

C

Figure 1. XRD patterns of MOFs grown on PP or FR fabric substrates. (A) XRD patterns of HKUST-1 powder, untreated PP, PP/ZnO(200), PP/ZnO(100)/HKUST-1, PP/ZnO(200)/HKUST-1, PP/ZnO(300)/HKUST-1. (B) XRD patterns of UiO-66-NH2 powder, untreated PP, PP/TiO2, and PP/TiO2/UiO-66-NH2. (C) XRD patterns of UiO-66NH2 powder, FR, FR/TiO2, FR/TiO2 after immersed in DMF at 85 ºC for 24 h, FR/TiO2 after immersed in DMF with HCl at 85 ºC for 24 h, and FR/TiO2/UiO-66-NH2.


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Figure 2. EDS mapping of FR/TiO2/UiO-66-NH2 obtained by SEM for (A) Carbon, (B) Oxygen, (C) Zirconium, and (D) Titanium.

The UiO-66-NH2 MOF was clearly present on the PP fibers and FR fabric. XRD patterns confirmed the presence of the MOF structure on the fibers at 7° and 8°, shown in Figure 1. While EDS confirmed the presence of Zr, shown in Figure 2, it would appear that the UiO-66-NH2 MOF growth was minimal on the FR fabric.

This was because the mass loading of UiO-66-NH2 onto FR fabric and onto PP fiber was 0.6 and 1.4 wt%, respectively. In addition, as shown in Figure 3b-c MOF, crystal size onto FR fabric was substantially smaller than that onto PP substrate. Peak broadening in the XRD pattern generally correlates to smaller crystal size, as shown in the Scherrer


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formula18. Therefore, the peaks for the FR fabric sample in Figure 1c were significantly smaller, less sharp and less pronounced than the peaks for the PP substrate.

It could have been that the UiO-66-NH2 MOF nucleated on the sensitized ALD surface but did not grow to the same extent as on the PP substrate, as interaction behavior between ALD precursors and polymeric surface are generally varied19.

SEM images of HKUST-1 and UiO-66-NH2 growth confirmed this, shown in Figure 3. UiO-66-NH2 had smaller crystal sizes, compared with HKUST-1. Additionally, crystal sizes of UiO-66-NH2 were smaller on the FR fabric than on PP.

Figure 3. SEM images of untreated PP and MOFs grown on ALD coated PP or FR fabric. Top row shows (A) Untreated PP, (B) UiO-66-NH2 grown on PP, and (C) UiO-66-


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NH2 grown on FR fabric. FR fabric and PP were coated with 300 cycles of ALD TiO2 before UiO-66-NH2 growth. Bottom row shows HKUST-1 grown on PP with (D) 100 cycles ZnO, (E) 200 cycles ZnO, or (F) 300 cycles of ZnO.

WATER VAPOR AND AIR TRANSPORT THROUGH PP/ZnO/HKUST-1 MATERIALS

The MVTR and air permeation of the PP/ZnO/HKUST-1 samples are shown in Table 1. MVTR and air permeation of 5 samples of uncoated PP were measured, averaged, and used as a baseline to assess the uniformity of the sample substrate. The thickness of the ZnO/HKUST-1 layers were varied in order to assess the effect on MVTR and air permeation. Within the range of material coating studied, the MVTR remained nearly the same after the ZnO and ZnO/HKUST-1 coating. Given the variability of the uncoated PP substrate, the MVTR remained relatively constant after depositing ZnO/HKUST-1 regardless of ZnO cycle number. This may be due to the high water loading achievable in HKUST-1 and a steady-state absorption-desorption process occurring once saturation occurs. Additionally, the ALD coating and MOF overlayers likely did not significantly impede moisture vapor transport because of the conformal morphology of the ALD and


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MOF coatings. Since the coatings were conformal and the fiber mats were flexible and free to expand, the coatings did not decrease the void space within the fiber mat.

Table 1. MVTR and air permeation of PP modified with ZnO and HKUST-1, with commercially available fabrics for comparison.

Sample

MVTR (g/m2/day)

Air permeation (cfm)

Uncoated PP

15,340 ±1,115

41.4 ± 4.4

PP/ZnO(200)

14,700

34.0

PP/ZnO(100)/HKUST-1

14,800

34.7

PP/ZnO(200)/HKUST-1

14,700

13.5

PP/ZnO(300)/HKUST-1

15,000

19.1

FR fabric

13,300

61.6

Carbon based material 1

4,300

12.1


8,700

160


10,600

43.8

Aerosol protective material 1

5,460

4.3

Aerosol protective material 2

5,550

6.5


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While the ZnO/HKUST-1 coating did not affect the MVTR, it did lead to a decrease in air permeation. Coating the PP with 200 cycles of ZnO alone produced an 18% decrease relative to the uncoated PP, and PP/ZnO(200)/HKUST-1 had a 67% decrease in air permeation from uncoated PP. The air permeation of PP/ZnO(200)/HKUST-1 and PP/ZnO(300)/HKUST-1 was approximately the same. It is likely there was a similar extent of HKUST-1 MOF coating on both these samples, due to less HKUST-1 growth than expected.

The MVTR and air permeation of select commercially available materials, shown in Table 1, are a standard of comparison to the novel materials developed in this work. The same materials were used previously to frame results20. High MVTR and air permeation is desirable, as it allows evaporative cooling and air movement through a garment. Air permeation of the carbon based materials was partly dependent upon substrate and material design; air permeation of the MOF coated materials appeared to be dependent upon MOF loading. The HKUST-1 coated materials were, in general, comparable to Carbon based material 3, though PP/ZnO(300)/HKUST-1 was more comparable to


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Carbon based material 1. Still, the MOF coated materials, even those with the highest MOF loadings, maintained air permeation values comparable to carbon based materials without affecting the MVTRs of the base substrate.

In order to assess whether the MVTR was affected by the sorption of water in the ZnO/HKUST-1 layers, the samples were preconditioned at room temperature at 97% RH for 24 hours and at 100% RH for 2 weeks. To compare humidified and dried conditions, samples were dried in a vacuum oven at 80 °C for 2 weeks. However, the water could not be baked off of the samples, evidenced by lack of an expected color change from blue to purple. Attempting to remove water from the samples by baking at higher temperatures resulted in severe thermal degradation of samples. Therefore, effect of water sorption on MVTR was compared between samples at high humidity and in ambient conditions. Table 2 summarizes the MVTR and air permeation measurements of humidified materials. Although the MVTR remained unchanged in comparison to the samples that were not preconditioned (see Table 1), the air permeation of the samples increased slightly in the case of the PP/ZnO, and very dramatically in the case of the PP/ZnO/HKUST-1. Indeed,


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after 2 weeks of humidification, the air permeation of PP/ZnO/HKUST-1 approached that of the substrate prior to the HKUST-1 growth. The decrease in air permeation was caused from the HKUST-1 crystal structure breaking down with increasing exposure time to humidity21.

Table 2. MVTR and air permeation of humidified PP/ZnO/HKUST-1 materials, with commercially available fabrics for comparison.

Sample

MVTR (g/m2/day)

Air permeation (cfm)

PP/ZnO(200) 97%RH/24 h

15,100

37.2

PP/ZnO(200) 100%RH/2 weeks

14,200

38.9

PP/ZnO(200)/HKUST-1 97%RH/24 h

15,100

17.6

100%RH/2 15,100

31.3

PP/ZnO(200)/HKUST-1 weeks

These sets of experiments show that increasing the amount of water vapor adsorbed on the ZnO/HKUST-1, even to the extent that the adsorbed water broke down the structure of the HKUST-1 MOF, did not affect MVTR. Although the water from the HKUST-1 could


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not be fully removed to measure the water vapor transport, one can assume that due to the affinity of the HKUST-1 and water21 the water vapor permeation would start off low and increase rapidly as the HKUST-1 reached steady state with the water vapor.

WATER VAPOR AND AIR TRANSPORT THROUGH PP/TiO2/UiO-66-NH2 MATERIALS

Table 3 shows the MVTR and air permeation of the TiO2/UiO-66-NH2 samples. Thickness of the TiO2/UiO-66-NH2 layers were not varied, but TiO2/UiO-66-NH2 was deposited onto either a PP microfiber substrate or FR fabric. As with the PP/ZnO/HKUST-1 composites, the MVTR remained constant as neither the TiO2 nor the UiO-66-NH2 affected MVTR. The use of these materials in CB protective textiles bodes well in terms of reducing the thermal burden of the wearer since the materials did not adversely affect MVTR.

The addition of ALD TiO2 decreased air permeation, compared to the uncoated substrates. Additionally, air permeation decreased as the MOF UiO-66-NH2 was added to the ALD TiO2 layer. However, there was no decrease in air permeation between


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FR/TiO2(300) and FR/TiO2(300)/UiO-66-NH2. This is thought to be due to a lack of growth of UiO-66-NH2 on the FR fabric, as indicated by the XRD and EDS spectra (Figure 1 and Figure 2, respectively).

The TiO2/UiO-66-NH2 composites decreased the air permeation in relation to the uncoated substrates, shown in Table 3, but not nearly as much as with ZnO/HKUST-1, shown in Table 1. One reason for this difference was that HKUST-1 crystal growth was larger than the UiO-66-NH2 crystal growth, as shown by SEM in Figure 3. Similarly, the 300 cycles deposition of TiO2 did not affect the air permeation as adversely as the 200 cycles deposition of ZnO because the ZnO deposition was greater than TiO2 deposition. Based on ellipsometry of monitor silicon, the layer thickness of ZnO(200) was 35 nm, while TiO2(300) was 17 nm thick.

Table 3. MVTR and air permeation of PP and FR fabrics modified with TiO2 and UiO-66NH2, with commercially available fabrics for comparison.

Sample

MVTR (g/m2/day) Air permeation (cfm)

Uncoated PP

15,340 ±1,114.9

41.4 ± 4.4


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FR/TiO2(300)

13,300

58.8

FR/TiO2(300)/UiO-66-NH2 12,500

58.8

PP/TiO2(300)

15,000

42.6

PP/TiO2(300)/UiO-66-NH2 15,700

32.7

In the case of the FR fabric, minimal air permeation reduction was observed after depositing TiO2, similar to the minimal impact on the PP, listed in Table 1. The subsequent deposition of the UiO-66-NH2 did not affect the air permeation at all, despite the evidence shown for the presence of the UiO-66-NH2 MOF in Figure 1 and Figure 2. This could likely be attributed to the lack of crystal growth on the FR fabric.

The FR fabrics had much higher air permeation values than any MOF coated materials, however, none of the FR fabrics provided any aerosol protection. The MOF coated materials and carbon based materials offered aerosol protection, in some cases correlating to a decrease in air permeation which will be discussed later. The UiO-66NH2 coated materials were comparable to Carbon based material 3, listed in Table 1. Still, all MOF coated materials, even those with the highest MOF loadings, maintained air


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permeation values comparable to carbon based materials without affecting the MVTRs of the base substrate.

AEROSOL FILTRATION EFFICIENCY OF PP/ZnO/HKUST-1 AND PP/TiO2/UiO-66-NH2 MATERIALS

The aerosol filtration efficiency of 0.05 – 0.4 DOP particles through the PP/ZnO/HKUST1 and PP/TiO2/UiO-66-NH2 materials is shown in Figure 4. In general, the filtration efficiency decreased from 0.05 µm to roughly 0.25 – 0.3 µm, and then increased with increasing particle size. The 0.25 – 0.3 µm particle size is generally observed to be the most penetrating particle size regardless of the material (22) and these composites exhibited the same behavior.

The PP, PP/ZnO(200) and PP/TiO2(300) had similar filtration efficiencies to one another, showing that the ALD deposition layer was not thick enough to improve the filtration efficiency of the PP microfiber substrate despite the reduction in air permeation seen with


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the addition of a ZnO or TiO2 layer. MOF growth did increase the filtration efficiency of the PP substrate, more so with the HKUST-1 (roughly 25% increase) than with the UiO66-NH2 (roughly 10% increase). These results correlate with the larger reduction seen in air permeation after HKUST-1 growth, as opposed to UiO-66-NH2 growth. Filtration efficiency increased only in the presence of MOF coatings. Increasing the number of ALD layers did not increase the filtration efficiency.

While ALD deposition layer was not thick enough to improve the filtration efficiency of the PP microfiber substrate, filtration efficiency was expected to increase with MOF layer thickness. Such a trend was not noted for the HKUST-1 materials. This was hypothesized to be due to less HKUST-1 growth than expected.

A

B


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Figure 4. (A) Filtration efficiency of PP/ZnO/HKUST-1 and PP/TiO2/UiO-66-NH2 materials. (B) Filtration efficiency of commercially available materials.

The filtration efficiencies of commercially available materials are shown in Figure 4 as a standard of comparison to the novel materials. These include two aerosol protective fabrics, carbon based material 1 and the FR fabric. The FR fabric is not designed to provide any aerosol protection, and the filtration efficiency is very low at roughly 10% for the particle sizes measured. Carbon based material 1, is also not designed to provide aerosol protection, although it has a very low air permeation. The filtration efficiency was roughly 35% for the particle sizes measured.

In comparison, all of the modified PP substrates had greater aerosol protection than the conventional carbon based protective fabric, and also retained a higher level of air permeation. The higher filtration efficiency with higher air permeation demonstrates the potential of using these MOF coated materials to increase aerosol protection while reducing the thermal burden to the wearer.


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The two aerosol protective materials made of polyurethane nanofibers are designed to have a very high filtration efficiency (aerosol protective material 1) or a mid-level filtration efficiency (aerosol protective material 2). The higher filtration efficiency material also has an extremely low air permeation (Table 1). The mid-level, commercially available material has roughly the same filtration efficiency as the PP/ZnO(200)/HKUST-1 at the most penetrating particle size, but again, the air permeation of the novel material was much higher.

CHEMICAL ACTIVITY

From a chemical protection standpoint, the substrates were evaluated for potential as both novel conformal filters and as highly sorptive suits. In the former, PP/HKUST-1 materials were evaluated against ammonia using microbreakthrough testing to understand the effect of ALD coating thickness on HKUST-1 growth and subsequent uptake. Ammonia was used as a probe not only because of its toxicity and inhalation hazard, but also because HKUST-1 is known to have extraordinarily high capacities. The breakthrough data are shown in Figure 5. Ammonia broke through the baseline PP fabric


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and reached saturation almost immediately, resulting in a loading of ~0.2 mol/kg. For MOF coated samples, as the amount of initial zinc oxide deposited on the surface increased, the resulting swatches provided enhanced ammonia uptake, ranging from 1.2 mol/kg for the material coated with 100 layers of zinc oxide to 2.2 mol/kg for the material coated with 300 layers of zinc oxide. This trend correlated well with the amount of MOF grown on the substrate as shown in the SEM images (Figure 3).

Whereas the

PP/ZnO(100)/HKUST-1 exhibited patchy growth, the materials grown with 200 and 300 layers exhibited conformal growth of high quality HKUST-1 crystals, therefore providing additional material for ammonia uptake and resulting in higher capacity swatches.

A

B

Figure 5. (A) Ammonia microbreakthrough of PP and PP/ZnO/HKUST-1 composites. (B) CEES permeation through PP/ZnO and PP/ZnO/HKUST-1 composites.


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CEES permeation data were collected on swatches to simulate protection afforded by a suit against a potential cutaneous exposure, because HKUST-1 is known to interact with CEES (6). Permeation data as a function of time are shown in Figure 5, and Table 4 summarizes the resulting permeation rates and CEES loadings on the swatches. As with ammonia, more ALD layers (and therefore more MOF growth) led to increased protection time, and the 300 layer material provided the most protection and lowest permeation rate over the time of the test. The amount of CEES taken up by the swatch also increased with increased ALD layers, which again correlated well with the amount of HKUST-1 grown on the fiber.

Table 4. CEES Permeation through MOF coated materials.

Sample

Steady-State Permeation Rate

CEES Loading

(µg/cm2/min)

(mol/kg)

PP/ZnO(100)/HKUST-1

2.863

0.209

PP/ZnO(200)/HKUST-1

2.740

0.271

PP/ZnO(300)/HKUST-1

2.545

0.307


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CONCLUSIONS AND FUTURE WORK

The work performed sought to define relationships between fiber surface modification with ALD coatings and the MOFs UiO-66-NH2 and HKUST-1. Crystal growth on PP or fabric substrates was correlated to MVTR, air permeation, filtration efficiency, and chemical activity.

MVTR was not affected by MOF growth on substrates. Substrate choice or fabric composition was observed to have a larger impact on MVTR than ALD deposition or MOF growth. Deposition of MOFs on substrates did not appear to impact MVTR at all, even in humidified environments. Comparisons to commercially available materials showed the potential advantages of MOF coated materials. Carbon based materials and FR fabrics in use by the military have relatively low MVTR and air permeation values, especially when compared with the MOF coated materials in this work. This was partly due to the base substrates chosen; PP is hydrophobic, the FR fabrics easily absorb water. It is also partly due to material design, such as carbon loading. The primary advantage of the MOF


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coated substrates demonstrated here was that the addition of MOFs did not impact the MVTR of the base substrate, even with higher MOF deposition.

FR fabrics had much higher air permeation values than any MOF coated materials, however, none of the FR fabrics provided any aerosol protection. The MOF coated materials and carbon based materials offered aerosol protection, in some cases correlating to a decrease in air permeation

ALD of ZnO or TiO2 with MOF growth did impact air permeation, with decreased air permeation associated with ALD layer thickness and MOF crystal size. MOF growth and ALD layer thickness likewise correlated with chemical activity, where ammonia uptake increased with larger and more conformal HKUST-1 crystal sizes. Chemical permeation of CEES was slowed both by MOF presence and number of ALD layers, with increasing CEES uptake correlating with increased MOF growth on thicker ALD layers. These results show that protective properties generally increase as air permeation decreased with increasing MOF growth and additional ALD layering.


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Higher filtration efficiencies were observed as air permeation decreased, when MOFs were present. Though ALD of ZnO and TiO2 decreased air permeation through substrates, it was not associated with increased filtration efficiency. The presence of either HKUST-1 or UiO-66-NH2 growth was required to see an increase in filtration efficiency. ALD layering and HKUST-1 MOF growth was varied to increase layer thickness on PP, but results did not follow the same trends as those observed in chemical sorption and permeation. Little difference was observed between air permeation and filtration efficiency of PP/ZnO(200)/HKUST-1 and PP/ZnO(300)/HKUST-1, even though both ALD layering and MOF growth increased. This was thought to be due to variability in MOF growth on the substrates. Both air permeation and filtration efficiency tests required larger swatch sizes, up to 5 inches in diameter, and variability in MOF growth on the fibers for these scaled up tests likely accounted for variability in these results.

To confirm the trends between MOF growth, sorption, and permeation, nitrogen isotherms should be measured in future work. Future work will also address the variability observed in larger swatch sizes, in order to optimize these methods for MOF deposition onto fiber


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or textile substrates at larger scales. In terms of utilizing MOF modified materials in CB protective garments, the lack of impact on MVTR is promising when considering thermal properties. Future work will continue to evaluate trends in MOF deposition and crystal size on transport properties and further explore how chemical sorbance and permeation is affected.

ASSOCIATED CONTENT

Supporting Information.

Supporting information to this article is available. This material is available free of charge at http://pubs.acs.org.

The supporting information includes scanning electron microscopy, moisture vapor transport (MVTR) and air permeation of polypropylene, and filtration efficiency tradespace.


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AUTHOR INFORMATION

Corresponding Author *Natalie Pomerantz, [email protected].

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Funding Sources This work was funded by the Defense Threat Reduction agency as part of the Multifunctional Materials for Chemical/Biological Protection (CB10187) and Multifunctional Materials for Force Protection (CB3934) Programs, Grant W911SR-07C-0075, Army Research Office Grant W911NF‐13‐1‐0173 and US Army Research Office DURIP Grant W911NF‐17‐1‐0166.


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ACKNOWLEDGMENT

The authors gratefully acknowledge the support of the Defense Threat Reduction Agency.

ABBREVIATIONS CB chemical/biological; ALD atomic layer deposition; MOF metal organic framework; HRMAS high resolution magic angle spinning; NMR nuclear magnetic resonance; DFP diisopropyl fluorophosphates; MVTR moisture vapor transport; DMPC dynamic moisture permeation cell; PP polypropylene; FR (flame resistant); XRD X-ray diffraction; SEM scanning electron microscopy; EDS electron dispersive spectrometry; CEES 2chloroethyl ethyl sulfide

REFERENCES


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TOC graphic


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Air, Water Vapor, and Aerosol Transport through Textiles with Surface

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